1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine which comprises a catalyst for purifying exhaust gases, and an adsorbent for temporarily adsorbing unburnt components in the exhaust gases and desorbing the once adsorbed unburnt components in an exhaust system.
2. Description of the Prior Art
A conventional exhaust gas purifying apparatus of the type mentioned above is known, for example, in Laid-open Japanese Patent Application No. 7-332074. The exhaust gas purifying apparatus described in this document comprises a catalyzer arranged in an exhaust pipe of an engine, and a hydrocarbon (HC) adsorbent arranged at a location downstream of the catalyzer in the exhaust pipe. The exhaust pipe has a main exhaust passage, and a bypass exhaust passage which is branched off the main exhaust passage at a location downstream of the catalyzer and joined again to the main exhaust passage at a location downstream of the branched location. In one implementation of this exhaust gas purifying apparatus, the main exhaust passage comprises a troidal passage portion which extends to completely surround the bypass exhaust passage. The troidal passage portion has a cross-section area substantially larger than that of the bypass exhaust passage. At an intermediate portion in the bypass exhaust passage, an HC adsorbent is arranged. This HC adsorbent is comprised of a honeycomb core, extending across the cross section of the bypass exhaust passage, which has a multiplicity of inner holes extending through the honeycomb core in the same direction along the bypass exhaust passage, and an HC adsorbent layer formed on the surfaces of the walls of the inner holes for adsorbing hydrocarbons. A flow path switching valve is arranged at the joint at which the bypass exhaust passage joins to the main exhaust passage. The flow path switching valve opens one of the main exhaust passage and the bypass exhaust passage while closing the other, thereby switching a flow pass of the exhaust gases from the catalyzer to the main exhaust passage or the bypass exhaust passage.
In the exhaust gas purifying apparatus described above, the flow path switching valve switches the exhaust gas flow path to the bypass exhaust passages at the time the engine is started, causing exhaust gases immediately after the starting of the engine to pass through the catalyzer and then pass through the HC adsorbent. In this event, hydrocarbons included in the exhaust gases immediately after the starting cannot be purified by the catalyzer, since it has not been activated due to a low temperature immediately after the starting, and therefore are adsorbed by the HC adsorbent when they pass therethrough. Subsequently, as the catalyzer is heated by the heat of the exhaust gases after the starting of the engine and eventually activated, the catalyzer starts purifying hydrocarbons in the exhaust gases, and the exhaust gas flow path is switched to the main exhaust passage by the flow path switching valve. This causes the exhaust gases to flow within the troidal passage along the outer peripheral surface of the HC adsorbent, so that the HC adsorbent is heated by heat exchange with the exhaust gases to desorb the adsorbed hydrocarbons. The desorbed hydrocarbons are recirculated to the catalyzer for purification.
In the conventional exhaust gas purifying apparatus described above, since the troidal passage of the main exhaust passage has a cross-sectional area significantly larger than that of the bypass exhaust passage, the heat of exhaust gases passing within the troidal passage is susceptible to dissipation from the troidal passage to the outside because the outer peripheral surface of the troidal passage has the area substantially larger than that of the outer peripheral surface of the bypass exhaust passage, when the exhaust gas flow path is connected to the main exhaust passage. This results in an inefficient heat exchange between the exhaust gases and the HC adsorbent, thereby preventing the HC adsorbent from being heated to a temperature sufficiently high to desorb hydrocarbons. Thus, the hydrocarbons remains in the HC adsorbent which can fail to recover its adsorbing capability. As a result, a smaller amount of hydrocarbons in the exhaust gases will be adsorbed by the HC adsorbent after the internal combustion engine is started next time, causing a degraded exhaust gas emission characteristic. Particularly, if hydrocarbons repeatedly remains in the HC adsorbent in this way, the accumulated hydrocarbons will gradually degrade the adsorbing capability of the HC adsorbent to prominently worsen the exhaust gas emission characteristic. While this problem may be solved by reducing the cross-sectional area of the troidal passage, such a reduction would result in an increased exhaust resistance in the troidal passage.
The present invention has been made to solve the problems as mentioned above, and its object is to provide an exhaust gas purifying apparatus for an internal combustion engine which is capable of simultaneously realizing sufficient recovery of the adsorbing capability of an adsorbent and a limitation to an increased exhaust resistance.
To achieve the above object, the present invention provides an exhaust gas purifying apparatus for an internal combustion engine, which is arranged in an exhaust system having a first exhaust passage connected to the internal combustion engine and a second exhaust passage having one end branched off the first exhaust passage and the other end joined to the first exhaust passage for purifying exhaust gases discharged from the internal combustion engine. The exhaust gas purifying apparatus includes switching means for switching an exhaust gas flow path to one of the second exhaust passage and the first exhaust passage in accordance with an operating state of the internal combustion engine; a catalyst for purifying exhaust gases in the exhaust system; and an adsorbent arranged in the second exhaust passage for adsorbing unburnt components in the exhaust gases supplied to the second exhaust passage through the switching means, and for desorbing the adsorbed unburnt components when the adsorbent is heated to a predetermined temperature or higher. The second exhaust passage is arranged in the first exhaust passage which includes a troidal passage which completely surrounds a portion of the second exhaust passage in which the adsorbent is arranged. With the exhaust gas flow path switched to the first exhaust passage by the switching means, a ratio of a cross-sectional area of the troidal passage to a cross-sectional area of a passage at a predetermined location of the exhaust system except for the troidal passage is set to a value in a range from a first predetermined value at which an exhaust resistance starts gradually decreasing as the ratio becomes higher to a second predetermined value at which the temperature of the adsorbent can rise to the predetermined temperature or higher.
According to the exhaust gas purifying apparatus for an internal combustion engine, the switching means switches the exhaust gas flow path to the first exhaust passage or to the second exhaust passage in accordance with an operating state of the internal combustion engine. By the action of the switching means, when the catalyst is not yet activated immediately after the internal combustion engine is started, the exhaust gas flow path is switched to the second exhaust passage such that unburnt components in the exhaust gases are adsorbed by the adsorbent in the second exhaust passage.
Also, when the exhaust gas flow path is switched to the first exhaust passage in synchronism with the activation of the catalyst after starting the internal combustion engine, the troidal passage of the first exhaust passage, which completely surrounds the portion of the second exhaust passage in which the adsorbent is arranged, acts to provide heat exchange between the exhaust gases flowing through the troidal passage and the adsorbent. In this event, generally, as the cross-sectional area of the troidal passage is smaller, the area ratio of the outer peripheral surface of the troidal passage to the outer peripheral surface of the second exhaust passage is smaller, so that the heat of the exhaust gases is less susceptible to dissipation to the outside of the troidal passage, and the exhaust gases are in contact with the outer peripheral surface of the second exhaust passage in a larger proportion, resulting in a tendency of an increased efficiency of the heat exchange between the exhaust gases flowing through the troidal passage and the adsorbent. Stated another way, as the ratio of the cross-sectional area of the troidal passage to the cross-sectional area at a predetermined location except for the troidal passage is lower, the adsorbent can reach a higher temperature due to the heat exchange with the exhaust gases. Thus, unburnt components adsorbed by the adsorbent can be desorbed by setting the ratio to a value equal to or less than a second predetermined value at which the temperature of the adsorbent is promptly increased to a full desorption temperature to desorb the adsorbed unburnt components, when the exhaust gas flow path has been switched to the first exhaust passage, with the result that the adsorbing capability of the adsorbent can be sufficiently recovered.
Also, since the exhaust resistance in an exhaust passage is generally smaller as the cross-sectional area of the exhaust passage is larger, the exhaust resistance tends to be smaller as the ratio of the cross-sectional area of the troidal passage to the cross-sectional area at a predetermined location of the first exhaust passage except for the troidal passage is larger. It is therefore possible to limit an increase in the exhaust resistance in the troidal passage by setting the ratio to a value equal to or more than a first predetermined value at which the exhaust resistance begins gradually decreasing as the ratio increases. In this way, according to the present invention, it is possible to simultaneously realize the sufficient recovery of the adsorbing capability of the adsorbent and limitation to an increase in the exhaust resistance by setting the ratio to a value in a range from the first predetermined value to the second predetermined value.
Preferably, in the exhaust gas purifying apparatus for an internal combustion engine, the predetermined location of the exhaust system is downstream of a portion from which the second exhaust passage is branched off the first exhaust passage and upstream of the troidal passage, the first predetermined value is set to 1.0, and the second predetermined value is set to 3.0.
When the predetermined location is selected to be downstream of a location from which the second exhaust passage is branched off the first exhaust passage and upstream of the troidal passage, i.e., at a location at which a flow path area is generally reduced to a minimum, it has been confirmed from experiments made by the Applicant that the first predetermined value is 1.0 and the second predetermined value is 3.0. With these settings, it is possible to simultaneously realize the sufficient recovery of the adsorbing capability of the adsorbent and limitation to an increase in the exhaust resistance.